The researchers said their National Institutes of Health-funded work might change the way physicians treat pain during and after surgery, as well chronic pain.

The scientists said they used specially designed fat-based particles called liposomes to package saxitoxin, a potent anesthetic, and produced long-lasting local anesthesia in rats without apparent toxicity to nerve or muscle cells.

“The idea was to have a single injection that could produce a nerve block lasting for days, weeks, maybe even sometimes months,” said Dr. Daniel Kohane, the report’s senior author. “It would be useful for conditions like chronic pain where, rather than use narcotics (that) are systemic and pose a risk of addiction, you could just put that piece of the body to sleep, so to speak.”

The scientists said that previous attempts to develop slow-release anesthetics have been unsuccessful due to toxicity problems. But in the new study, Kohane and his colleagues report saxitoxin packaged within liposomes is able to block nerve transmission of pain without causing significant nerve or muscle damage.

Kohane said he is now optimizing the formulation to make it last even longer and it is quite conceivable that clinical trials would soon start.

The research appears in the online edition of the Proceedings of the National Academy of Sciences.